1. Field
The present disclosure relates to lock systems for securing access to manhole openings. More particularly, the disclosure concerns a self-locking manhole cover.
2. Description of Prior Art
By way of background, standard manhole covers are designed to be easily removed from manhole openings to allow access to underground facilities such as sewers, electrical and communication equipment vaults, and other infrastructure. This presents a security risk by allowing vandals, terrorists and others to gain unauthorized access to important assets, or to move about undetected via underground passageways. Standard manhole covers are also attractive targets for thieves who sell the covers for their scrap metal value.
Various manhole locking schemes have been proposed to address such security concerns. One technique is to simply bolt the manhole cover to the underlying manhole frame structure. Although very effective, this method involves retrofitting existing manhole covers and frames by drilling and tapping bolt holes, or requires that existing covers and frames be replaced with units having preformed bolt holes. Both alternatives are labor intensive and may be prohibitively expensive if the number of manhole locations is large.
Another manhole security technique involves the use of a lockable pan unit situated below a standard manhole cover. The pan unit is used to block the manhole opening, which means that the manhole cover itself does not require locking and does not have to be retrofitted or replaced. The pan unit is secured to the manhole frame by resting it on the same support surface that supports the manhole cover (typically a ring flange), and then locking the unit to the manhole frame. A disadvantage of such systems is the requirement for a separate pan that must be separately removed after the manhole cover is removed. Moreover, this solution does not prevent manhole cover theft.
Another manhole security technique involves providing a lock system on the manhole cover itself. A typical lock system includes a pair of retractable lock rods or bars that extend horizontally to engage the side-wall of the manhole frame or the underside of the ring flange or other support surface that supports the manhole cover. A rotatable key is used to rotate a locking apparatus or actuator that actuates the rods or bars into and out of locking engagement. By way of example, U.S. Pat. No. 4,964,755 discloses a manhole cover wherein a lock apparatus is turned by a key to operate a pair of lock rods. However, the lock rods are not self-locking and the key must be used to return the rods to their locked position once the manhole cover is in place. Moreover, the position of the lock rods in the locked position is fixed. Due to dimensional tolerances and differences between manhole frame designs, the lock rods may not firmly engage some manhole frames or may be overly tight in other manhole frames. U.S. Pat. No. 5,082,392 overcomes this problem by spring-biasing a pair of locking bars to their locked position. The locking bars affirmatively engage the manhole frame under the force of the biasing springs. A specially configured portion of a key mates with a vent hole in the manhole cover when the locking bars are in their unlocked position. This allows the locking bars to be held in the unlocked position during opening and closing of the manhole opening. However, the key must remain engaged with the manhole cover at all times when the cover is not covering the manhole, which may be inconvenient.
Applicants have observed a further disadvantage of existing locking manhole covers, namely, that such covers are susceptible to environmental degradation due to contact with sewer gases or other caustic agents. These harsh materials can corrode and degrade the cover locking components, thereby increasing service and repair costs.
It is to improvements in manhole opening security systems that the present disclosure is directed. In particular, applicants have perceived a need for a security device that improves upon previous designs by reducing the effort required to lock and unlock the device, that provides robust locking capability using an uncomplicated design that is easy to manufacture, and which is corrosion-resistant.